In dense wavelength division multiplexing (DWDM) fiberoptic systems, multiple separate data streams propagate concurrently in a single optical fiber, with each data stream created by the modulated output of a laser at a specific channel frequency or wavelength. Channel separations of approximately 0.4 nanometers in wavelength, or about 50 gigahertz (GHz) are achievable, which allow up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.
DWDM systems have largely been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels.
Continuously tunable external cavity diode lasers (ECDLs) have been developed and are gaining their popularity due to superior optical performance, high reliability, and high manufacturing yield. External cavity lasers must be able to provide a stable, single mode output at selectable wavelengths while effectively suppress lasing associated with all other external cavity modes that are within the gain bandwidth of the cavity, which can be difficult to achieve.
A typical ECDL includes a gain medium diode with an antireflection-coated facet and a reflective or partially reflective facet, an end mirror, and a wavelength selection element (optical filter). The end mirror and reflective facet form an external laser cavity. The parameters of the gain medium are usually chosen to maximize the output power of the ECDL. The output power of the ECDL is maximized by, among other things, increasing gain and operating current of the gain medium. Typically, output power values attainable by ECDLs are in the range of 10 to 20 milliwatts (mW).
Although ECDL's usually provide higher optical power than integrated transmitters such as DFB lasers and distributed Bragg reflector (DBR) lasers, their output power is still a factor of two lower than what is demanded by some network builders. This power limitation is dictated by mode stability requirement and reliability concerns. Increasing a gain medium's gain and operating current to maximize ECDL output power may cause ECDL may become unstable in single mode and begin multimode lasing. In other words, the ECDL begins lasing at different frequencies simultaneously or hops between several frequencies. As described above, when the gain medium is used in an application such as a tunable ECDL in an optical communication system this instability may be unacceptable.